1. Technical Field of the Invention
The present invention relates to cellular telecommunications networks and, in particular, to retuning cells within telecommunications networks.
2. Description of Related Art
Reference is now made to FIG. 1 wherein there is illustrated an exemplary cell structure and frequency plan assignment for use in a radio frequency reuse cellular telephone system. An arbitrary geographic region (hereinafter xe2x80x9cthe service areaxe2x80x9d) is divided into a plurality of contiguous cell sites 10 schematically represented by hexagons. Each of these cell sites 10 is sectorized to include three cells 11. The cell sites 10 are then grouped into clusters 12 (outlined in bold to ease recognition). For example, in the frequency plan of FIG. 1, each cluster 12 includes seven cell sites 10(1)-10(7). It will, of course, be understood that each cluster 12 may have more or less cell sites 10 as required by the selected frequency plan.
The available frequencies in the allocated hyperband are divided in accordance with the frequency plan into channel groups 14, with the channel groups assigned amongst the cells 11 of each cell site 10 in a cluster 12 such that the radio channels of the allocated hyperband are reused in each cluster. For example, in a plan having seven cell sites 10 per cluster 12 (and three cells 11 per cell site) like that shown in FIG. 1, there are twenty-one channel groups 14 identified and differentiated from each other by the alphanumeric labels xe2x80x9cA1, A2, A3xe2x80x9d through xe2x80x9cG1, G2, G3xe2x80x9d corresponding to the cells 11(1)a-11(7)c, respectively. Thus, each cell 11(1)a in the service area is assigned use of radio channels of the allocated hyperband in channel group Al, each cell 11(2)a is assigned use of radio channels of the allocated hyperband in channel group B1, and so on up to each cell 11(7)a being assigned use of radio channels of the allocated hyperband in channel group G1. Cells 11(1)b through 11(7)b are assigned the radio channels of the allocated hyperbands in channel groups A2 through G2, respectively, while cells 11(1)c through 11(7)c are assigned the radio channels of the allocated hyperbands in channel groups A3 through G3, respectively. An exemplary channel chart correlating three hundred thirty-three available individual analog radio channels to the channel groups 14 in the plan illustrated in FIG. 1 is shown in TABLE 1.
It will be noted that in such a frequency plan, adjacent cells 11 are typically not assigned use of the same channel group. Reuse of an identical channel group in the service area is preferably made with a separation of at least more than one cell site 10 along with a regulation of broadcast power from each cell to constrain radio propagation substantially within the cell area. Furthermore, it will be noted that typically no one cell 11 utilizes channels in the allocated hyperband that do not meet some minimal channel separation. Adjacent channel groups are preferably assigned no closer than one cell 11 away from each other. By arranging the cell sites 10 in clusters 12 and dividing cell sites into cells 11 as shown in FIG. 1, regulating broadcast power of communications within the cell as mentioned above, and further by assigning channels in the fashion mentioned above, the likelihood of interference is reduced while simultaneously providing effective cellular communications services across a very large service area.
Each of the cell sites 10 in a cellular telephone system such as that illustrated in FIG. 1 includes at least one base station (BS) 18 configured to facilitate radio channel communications with mobile stations 20 moving throughout the service area. The base stations 18 are illustrated as being positionally located at or near the center of each of the cell sites 10. However, depending on geography and other known factors, the base stations 18 may instead be located at or near the periphery of, or otherwise away from the centers of, each of the cell sites 10. Each base station 18 includes three directional antennas 19, wherein each of the directional antennas are pointed towards and serve a particular cell 11.
The base stations 18 are connected by communications links (generally shown by arrow 16) to at least one mobile switching center (MSC) 22 operating to control the operation of the system for providing cellular communications with the mobile stations 20. Operation of the mobile switching center 22 and base station 18 to provide cellular telephone service is well known to those skilled in the art, and will not be described.
Referring now to FIG. 2, an exemplary base station 18 configured in accordance with the present invention is described. Each base station 18 includes three directional antennas 19a, 19b, and 19c, each serving a particular cell 11. Each antenna is connected to a plurality of channel equipments (Tx/Rx) 24 each capable of operating independently on a different radio channel of the channel group 14 assigned to the cell 11 served by the antenna 19. In the present example, each channel equipment 24 connected to directional antenna 19a operates at a different radio channel of the A1 channel group as described in TABLE 2.
The capacity or maximum number of users of a cell site 10 is dictated by the number of assigned channels. In order to accommodate the demand for a cellular telecommunications network, each cell site 10 is strategically placed and sized so that the demand within the cell site is commensurate with the capacity. Accordingly, in high traffic areas such as a metropolitan city, cell sites 10 are generally smaller in size, while in more rural areas, cell sites 10 are larger in size. After the cell sites 10 are strategically positioned and sized, channel assignments are made in accordance with a frequency reuse plan which minimizes co-channel interference.
However, over time the demand for cellular telecommunications services is subject to change. Increasing numbers of persons subscribing to cellular services as well as population growth in the geographic area of the cell site 10 can cause the demand for cellular telecommunications services to exceed the capacity of a cell site 10. To accommodate the additional demand, additional cell sites 10 are placed in the area and a larger number of smaller cell sites 10 accommodate the same area. The placement of additional cell sites 10 requires a reassignment of the channels for the cells 11 to minimize co-channel interference. The reassignment often requires preexisting cells 11 to be retuned in accordance with the channel reassignment. When the cell 11 is retuned to a new channel assignment, each channel equipment 24 connected to the directional antenna 19 serving the cell must be retuned to one of the new channels. The process of retuning a channel equipment 24 involves temporarily disabling the channel equipment and calibrating the channel equipment to a new channel.
A problem arises when a channel equipment 24 is retuned to a channel that is too close to a channel currently used by another channel equipment 24. For example, if the channel equipment 24 connected to the directional antenna 19a were to be retuned from the A1 channel group to the G3 channel group, each channel equipment would have to be retuned from a channel of the A1 channel group to a channel of the G3 channel group as shown in TABLE 3.
In particular channel equipment 24(1) must be retuned from channel 333 to channel 313. However, channel equipment 24(2) operates at channel 312 pursuant to the A1 channel group. Therefore, if channel equipment 24(1) is retuned to channel 313 while channel equipment 24(2) is still operating at channel 312, the channel separation between the channel equipment 24(1) and channel equipment 24(2) would be too small and likely to introduce interference concerns. Furthermore, the power calibration on the retuned channel equipment 24(1) is affected and the channel equipment 24(1) may be wrongly tuned as a result.
Although simultaneously retuning each channel equipment 24 would prevent the aforementioned problem, a simultaneous retune of all channel equipment in a cell 11 would require temporarily disabling all the channel equipment. In this retune scenario, all currently pending calls within the cell 11 would be dropped. Therefore, it would be preferred if the retune could be implemented, at least to some extent, in serial fashion in order for all calls to be maintained, albeit the reduced capacity service provision within the cell 11.
The present invention is directed to a system and method for retuning channel equipment. The channel equipment are first sorted in an order for retune from first to last wherein the target channel for each channel equipment maintains a threshold channel separation from the current channels for the channel equipment that are sorted later in the order for retune. The channel equipment are then retuned in accordance with the sorted order.